1. Field of the Invention
The present invention relates to a method and apparatus for scheduling multiple users in a communication system. More particularly, the present invention relates to a method and apparatus for scheduling multiple user terminals depending on partial channel information in a Multiple Input Multiple Output (MIMO) based communication system using multiple transmit/receive antennas.
2. Description of the Related Art
In a MIMO communication system including M transmit antennas, N receive antennas and K user terminals, when there is only a K=1 single user terminal, a multiplexing gain of the transmit antennas and the receive antennas is proportional to a minimum value min(M,N) between M and N. On the other hand, when there are K≧2 multiple user terminals, multiuser interference occurs between user terminals (hereinafter referred to as ‘multiuser terminals’) which are multiplexed with the transmit antennas. Therefore, a multiplexing gain is proportional to a minimum value min(M,K) between M and K. Thus, various methods have been proposed which perform multiplexing while canceling the multiuser interference between multiuser terminals.
FIG. 1 is a diagram illustrating a conventional configuration of a MIMO communication system.
Referring to FIG. 1, a base station 100 includes four transmit antennas 102 ˜108, and user terminal #1 110 through user terminal #4 116 that each include one receive antenna.
For example, for the transmit antenna 102, when one multiplexing channel is established with the user terminal #1 110, interference to the other user terminal #2 112 through terminal #4 116 occurs. Similarly, for each of the transmit antennas 102 ˜108, when one multiplexing channel is established to one associated user terminal, interference occurs to the remaining three user terminals except for the user terminal with which the multiplexing channel is established.
A Dirty Paper Coding (DPC) method has been proposed to cancel interference between a base station and user terminals in the MIMO communication system of FIG. 1. With the DPC method, optimal multiplexing is performed while canceling the multiuser interference between multiuser terminals using antenna channel information of all user terminals. However, it is difficult to apply the DPC method to a real-world system, since the system implementing the DPC method would need the antenna channel information of all of the user terminals.
Partial Channel State Information (PCSI) has been proposed to address the deficiencies of the DPC method. PCSI includes Signal to Interference plus Noise Ratio (SINR), best preceding vector index, etc. A random beamforming scheme based on PCSI selects users with a SINR greater than a threshold among a plurality of user terminals and transmits data thereto. The random beamforming scheme based on PCSI shows performance approximating that of the DPC method during multiplexing in the case where there is a large number of user terminals.
The PCSI scheme uses a plurality of random precoding vectors, the number of which corresponds to the number of transmit antennas. Specifically, a transmission signal of a base station is transmitted to a corresponding user terminal over a channel after being multiplied by a preceding signal. Each user terminal then calculates the SINR of a received channel to generate feedback information using an index of a preceding vector having the maximum SINR and an SINR of the preceding vector. The feedback information is then fed back to the base station. Thereafter, the base station, which has received the feedback information, selects, as multiuser terminals, a user terminal with the maximum SINR separately for each preceding vector. The base station then performs multiplexing using the selected multiuser terminals. In this case, if the number of users is insufficient, because the number of simultaneously transmitting users is equal to the number of transmit antennas, a multiplexing gain (throughput) is advantageously proportional to the number of transmit antennas of the terminal. However, the number of user terminals, which is needed to obtain a multiplexing gain proportional to the number of transmit antennas, exponentially increases with the number of transmit antennas. On the other hand, in a real-world environment where the number of user terminals is limited, because the number of multiuser terminals is equal to the number of transmit antennas, it is difficult to find a user terminal capable of avoiding inter-user interference. Therefore, an influence of the increase in interference increases with the number of multiuser terminals, causing a rare increase or an unexpected decrease in the throughput achieved by multiplexing.